There's no way they could have defeated the Empire. The Mon Cal cruisers provided the heavy punch needed to truly take on the Empire. The Rebels tactic to take out Star Destroyers was to isolate them, send in fighters, and pound the crap out of 'em with torpedos. Mon Cal cruisers and their veteran crews could go 1-on-1 with Star Destroyers. B-Wings allowed for more assaults on the Empire instead of quick hit-and-run attacks. Combined with good leadership, these things made possible the destruction of Death Star II, the capture of Coruscant, the defeat of Thrawn, etc.